Origin of Ultralow Thermal Conductivity in Metal Halide Perovskites

Resulting from their remarkable structure-property relationships, metal halide perovskites have garnered tremendous attention in recent years for a plethora of applications. For instance, their ultralow thermal conductivities make them promising candidates for thermoelectric and thermal barrier coating applications. It is widely accepted that the "guest" cations inside the metal halide framework act as "rattlers", which gives rise to strong intrinsic phonon resistances, thus explaining the structure-property relationship dictating their ultralow thermal conductivities. In contrast, through systematic atomistic simulations, we show that this conventionally accepted "rattling" behavior is not the mechanism dictating the ultralow thermal conductivities in metal halide perovskites. Instead, we show that the ultralow thermal conductivities in these materials mainly originate from the strongly anharmonic and mechanically soft metal halide framework. By comparing the thermal transport properties of the prototypical fully inorganic CsPbI₃ and an empty PbI₆ framework, we show that the addition of Cs⁺ ions inside the nanocages leads to an enhancement in thermal conductivity through vibrational hardening of the framework. Our extensive spectral energy density calculations show that the Cs⁺ ions have well-defined phase relations with the lattice dynamics of the "host" framework resulting in additional pathways for heat conduction, which is in disagreement with the description of the individual "rattling" of guests inside the framework that has been widely assumed to dictate their ultralow thermal conductivities. Furthermore, we show that an efficient strategy to control the heat transfer efficacy in these materials is through the manipulation of the framework anharmonicity achieved via strain and octahedral tilting. Our work provides the fundamental insights into the lattice dynamics that dictate heat transfer in these novel materials, which will ultimately help guide their further advancement in the next-generation of electronics such as in thermoelectric and photovoltaic applications.